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Sites pulling sneaky Flash cookie-snoop

Academics fret over privacy threat

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Many websites are using Flash-based cookies to track users, but often omit to mention this in their privacy policies.

US academics have documented the little-known tracking technology and its use in practice in a paper called Flash Cookies and Privacy. Browser-based cookies constitute a well understood and widely deployed technology that poses serious questions about privacy, depending on its usage.

What's far less well known is that Adobe Flash software also features cookies that can be used in much the same way as HTTP cookies. Flash cookies can be used for storing the volume level of a Flash video but the technology can also be used as "secondary, redundant unique identifiers that enable advertisers to circumvent user preferences and self-help", the academics warn.

A significant percentage of websites including federal government sites use this Flash-based technology to track users, the researchers discovered. The technology is sometimes used as a means to "undelete" the information in browser-based cookies that a user might have thought they had cleared from their system when they deleted their browsing history, the academics explain.

We find that more than 50 per cent of the sites in our sample are using flash cookies to store information about the user. Some are using it to 'respawn' or re-instantiate HTTP cookies deleted by the user. Flash cookies often share the same values as HTTP cookies, and are even used on government websites to assign unique values to users. Privacy policies rarely disclose the presence of Flash cookies, and user controls for effectuating privacy preferences are lacking.

The researchers conclude that Flash cookies are more effective at tracking users' visits around websites than traditional HTTP cookies because they operate in the shadows and are infrequently removed. By default Flash cookies have no built-in expiration date. Browser-based actions such as deleting browser histories or switching to private mode does not affect the operation of Flash cookies.

Separate studies suggest 30 per cent of users delete HTTP cookies once a month, creating difficulty in identifying unique visitors. Flash-based cookies increase the reliability of tracking methods and are therefore of interest to online advertisers. Third-party advertising networks were the most common source of Flash cookies that share the same values as HTTP cookies.

As part of the pilot study, the researchers also searched for mention of Flash cookies in the top 100 websites, encountering the term only four times. In some cases - such as Whitehouse.gov, which uses Flash cookies - the term 'tracking' is used without going into details, but the academics suspect not even a hint about the use of the technology is included in many cases.

Anti-tracking tools effective in blocking Flash-based cookies are far from widespread. Cookies can be deleted in Flash player itself, but this is a separate process from deleting them in a browser.

The researchers call for incorporation of Flash-cookie deletion into browsers and the inclusion of discussions about Flash cookies in the wider online privacy debate, which has recently concentrated on behavioural-targeting technology.

The research was carried out by a team of five researchers including Ashkan Soltani for the computing department at University of California, Berkeley and Chris Jay Hoofnagle from Berkeley's school of law. Shannon Canty from Clemson University, Quentin Mayo from Jacksonville State University and Lauren Thomas from Louisiana State University made up the remaining three members of the team. ®